Tetracalcium phosphate (TTCP) with surface whiskers and method of making same

ABSTRACT

A tetracalcium phosphate (TTCP) particle for use in preparing a fast-setting, bioresorbable calcium phosphate cement is disclosed. The TTCP particle has a basic calcium phosphate whiskers or fine crystals on a surface thereof; the basic calcium phosphate whiskers of fine crystals having a Ca/P molar greater that 1.33, and having a length up to about 5000 nm and a width up to about 500 nm.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation application and claims the benefit of priority under 35 USC § 120 to, U.S. patent application Ser. No. 10/944,278, filed Sep. 17, 2004 now U.S. Pat No. 7,066,999, which is a continuation of U.S. patent application Ser. No. 10/328,019 filed Dec. 26, 2006, now U.S. Pat. No. 6,840,995, which is a continuation-in-part application of U.S. patent application Ser. No. 09/615,384, filed Jul. 13, 2000, now abandoned. The above-listed applications are commonly assigned with the present invention and the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a tetracalcium phosphate (TTCP) for producing fast-setting, bioresorbable calcium phosphate cements (CPC), and in particular, to a tetracalcium phosphate having whiskers or fine crystals on the surface thereof for producing fast-setting, bioresorbable CPC having a high initial strength.

2. Description of the Related Art

U.S. Pat. No. 6,379,453B1 which is commonly assigned with the present invention discloses a process for producing a fast-setting, bioresorbable calcium phosphate cement comprising the following steps: obtaining a powder mixture from at least one calcium phosphate selected from the group consisting of Ca₄(PO₄)₂O, Ca(HPO₄)₂H₂O, CaHPO₄, Ca₈H₂(PO₄)₆.5H₂O, α-Ca₃(PO4)₂, β-Ca₃(PO₄)₂, Ca₂P₂O₇, Ca₂H₂P₂O₈, wherein the molar ratio of Ca to P in the mixture is roughly between 1 and 2; mixing the powder mixture in a phosphate-containing solution to obtain a powder/solution mixture having a concentration of less than 4 g powder mixture per ml solution; immediately heating the powder/solution mixture to a temperature of roughly 50° C.–350° C. to obtain a powder containing uniformly distributed submicron-sized apatite crystals; and mixing the apatite crystal-containing powder in a phosphate ion-containing solution to obtain a fast-setting, bioresorbable calcium phosphate cement.

SUMMARY OF THE INVENTION

An extensive study on the preparation of the fast-setting, bioresorbable calcium phosphate cement disclosed in U.S. Pat. No. 6,379,453 B1 has been conducted by the same inventors and their co-workers, and found that a fast-setting, bioresorbable CPC having a high initial strength can be prepared from a unique calcium phosphate, tetracalcium phosphate (Ca₄(PO₄)₂O, TTCP) particle having basic whiskers or fine crystals on the surface thereof, wherein said basic Whiskers or fine crystals have a Ca/P ratio greater than 1.33. Therefore an object of the invention is to provide such a unique TTCP particle. Another object of the present invention is to provide a process for preparing said unique TTCP particle. A further object of the present invention is to provide a fast-setting, bioresorbable CPC calcium phosphate cement prepared from said unique TTCP particle.

The invention accomplishes the above object by providing a tetracalcium phosphate (Ca₄(PO₄)₂O, TTCP) particle having basic whiskers or fine crystals on the surface, basic whiskers or fine crystals having a Ca/P ratio greater than 1.3, and preferably greater than 1.35 and less than 4.0. Said whiskers or fine crystals have a length up to about 5000 nm and a width up to about 500 nm, and preferably, a length from about 1 nm to about 2000 nm and a width from about 1 nm to about 200 nm. Said basic whiskers or fine crystals comprises TTCP as a major portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a to 1 c are related to microstructure and diffraction pattern of calcium phosphate whiskers grown on TTCP surface according to the present invention, wherein (a) bright field image of whiskers; (b) electron diffraction pattern of whiskers; and (c) interpretation of the diffraction pattern.

FIG. 2 shows XRD patterns, wherein (a) TTCP without whisker treatment; (b) TTCP with whisker treatment in (NH₄)₂HPO₄ for 5 minutes; and (c) CPC prepared from whisker-treated TTCP powder immersed in Hanks' solution for 24 hours.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A suitable method for preparing tetracalcium phosphate (TTCP) particles having a basic calcium phosphate whiskers or fine crystals on the surfaces of said TTCP particles of the present invention, said basic calcium phosphate whiskers or fine crystals having a Ca/P molar ratio greater than 1.33, comprises the following steps:

-   -   (a) obtaining TTCP particles;     -   (b) mixing said TTCP particles with a whisker-inducing solution,         allowing basic calcium phosphate whiskers or fine crystals         having a Ca/P molar ratio greater than 1.33 to grow on the         surfaces of said TTCP particles; and     -   (c) drying the whisker/fine crystal-grown particles.

Optionally, at least one additive selected from the group consisting of sodium phosphate (Na₃PO₄), disodium hydrogen phosphate (Na₂HPO₄), sodium dihydrogen phosphate (NaH₂PO₄), disodium hydrogen phosphate dodecahydrate (Na₂HPO₄.12H₂O), disodium hydrogen phosphate heptahydrate (Na₂HPO₄.7H₂O), sodium phosphate dodecahydrate (Na₃PO₄.12H₂O), orthophosphoric acid (H₃PO₄), calcium sulfate (CaSO₄), Ca₄(PO₄)₂O, CaHPO₄.2H₂O, CaHPO₄, Ca₈H₂(PO₄)₆.5H₂O, α-Ca₃(PO₄)₂, β-Ca₃(PO₄)₂, Ca₂P₂O₇, and Ca₂H₂O₈, (NH₄)₃PO₄, (NH₄)₂HPO₄, and (NH₄)H₂PO₄ together with said TTCP particles are mixed with the whisker-inducing solution in step (b).

Optionally, step (c) comprises separating the whisker-inducing solution and the TTCP particles, and heating the separated TTCP particles to a temperature up to about 1000° C., preferably about 50 to 500° C., to allow the basic calcium phosphate whiskers or fine crystals to grow to a length up to about 5000 nm and a width up to about 500 nm on the surfaces of said TTCP particles.

The heating includes (but not limited to) the conventional oven/furnace heating, resistance heating, infrared heating, microwave heating, electron beam heating, ion beam heating, laser beam heating and plasma heating. Preferably said heating is conducted in vacuum, inert atmosphere or air atmosphere.

The whisker-inducing solution in step (b) may be an acidic aqueous solution, a basic aqueous solution, an organic solvent or a substantially pure water. The acidic aqueous solution may contain at least one Ca or P source, or is free from Ca and P. The acidic aqueous solution can be selected from the group consisting of nitric acid (HNO₃), hydrochloric acid (HCl), phosphoric acid (H₃PO₄), carbonic acid (H₂CO₃), sodium dihydrogen phosphate (NaH₂PO₄), sodium dihydrogen phosphate monohydrate, sodium dihydrogen phosphate dihydrate, potassium dihydrogen phosphate (KH₂PO₄), ammonium dihydrogen phosphate (NH₄H₂PO₄), malic acid, acetic acid, lactic acid, citric acid, malonic acid, succinic acid, glutaric acid, tartaric acid, oxalic acid and their mixture.

The basic aqueous solution for use as the whisker-inducing solution in the method of the present invention may contain at least one Ca or P source, or is substantially free from Ca and P. The basic aqueous solution may be selected from the group consisting of ammonia, ammonium hydroxide, alkali metal hydroxide, alkali earth hydroxide, disodium hydrogen phosphate (Na₂HPO₄), disodium hydrogen phosphate dodecahydrate, disodium hydrogen phosphate heptahydrate, sodium phosphate dodecahydrate (Na₃PO₄.12H₂O), dipotassium hydrogen phosphate (K₂HPO₄), potassium phosphate tribasic (K₃PO₄), diammonium hydrogen phosphate ((NH₄)₂HPO₄), ammonium phosphate trihydrate ((NH₄)₃PO₄.3H₂O), sodium bicarbonate (NaHCO₃), and their mixture.

The present invention also discloses a calcium phosphate cement (CPC) powder comprising the TTCP particles of the present invention.

The following examples are intended to demonstrate the invention more fully without acting as a limitation upon its scope, since numerous modifications and variations will be apparent to those skilled in this art.

TTCP Preparation

The TTCP powder was fabricated in-house from the reaction of dicalcium pyrophosphate (Ca₂P₂O₇) (Sigma Chem. Co., St. Louis, Mo., USA) and calcium carbonate (CaCO₃) (Katayama Chem. Co., Tokyo, Japan) using the method suggested by Brown and Epstein [Journal of Research of the National Bureau of Standards—A Physics and Chemistry 6 (1965) 69A 12].

TEM Examination

A Hitachi Model-HF2000 200 kV field emission transmission electron microscope (TEM) equipped with a Noran Vayager Model 1000 energy dispersive spectroscopy (EDS) system was used for the study. The aperture size for microchemical analysis (Ca/P ratio) is 15 nm.

[NOTE: All “average” Ca/P ratios are averages of 10 measurements]

EXAMPLE 1

Whisker-Inducing Treatment of TTCP Particles Treated in Phosphate-Containing Basic Solution

Ca₄(PO₄)₂O (TTCP) powder as synthesized was sieved with a #325 mesh. The sieved powder has an average particle size of about 10 μm. An aqueous solution of diammonium hydrogen phosphate was prepared by dissolving 20 mg of diammonium hydrogen phosphate, (NH₄)₂HPO₄, in 40 ml deionized water. The resulting solution had a pH value of 8.02. To the TTCP powder the basic aqueous solution of diammonium hydrogen phosphate was added according to the ratio of 1 gm TTCP/13 ml solution. The TTCP powder was immersed in the basic aqueous solution for various periods of time of 1, 5 and 10 minutes, filtered rapidly and washed with deionized water, and filtered rapidly with a vacuum pump again. The resulting powder cake was dried in an oven at 50° C. The dried powder was dispersed in ethanol with supersonication. A drop of the dispersion was dripped on a single-side carbon sieve of #325 mesh having a diameter of 3 mm, and left dry to obtain a specimen coated with a thin carbon film for electrical conductivity for TEM examination.

Results:

-   (a) treated for 1 min: Whiskers grown on TTCP surface are acidic     (Ca/P<1.33) in nature. The EDS-determined Ca/P molar ratios are     between about 1.0 and 1.3 with an average Ca/P ratio of about 1.2.     Majority of whiskers have lengths <200 nm and widths <100 nm. -   (b) treated for 5 min: Whiskers grown on TTCP surface are basic     (Ca/P>1.33) in nature. The EDS-determined Ca/P molar ratios are     between about 1.4 and 1.9 with an average Ca/P ratio of about 1.6.     Majority of whiskers have lengths <200 nm and widths <100 nm. -   (c) treated for 10 min: Whiskers grown on TTCP surface are basic     (Ca/P>1.33) in nature. The EDS-determined Ca/P molar ratios are     between about 1.4 and 3.8 with an average Ca/P ratio of about 2.1.     Majority of whiskers have lengths <300 nm and widths <100 nm.

FIG. 1 represents a typical microstructure of the calcium phosphate whiskers grown on TTCP surface under such condition. FIG. 1( a) is a bright-field image showing the whiskers are substantially radial-oriented and the majority of which have lengths <300 nm and widths <100 nm; FIG. 1( b) is a typical electron diffraction pattern of such whiskers. The dotted-ring pattern is a direct result of the diffraction of numerous nano-sized whiskers; FIG. 1( c) is the indexing/interpretation of the diffraction pattern, which clearly shows that every ring matches a certain crystallographic plane of TTCP phase, indicating the whiskers have a TTCP crystal structure. The absence of HA (100) ring (d=0.817 nm) in the diffraction pattern excludes the possibility for the whiskers to have an apatite crystal structure under this whisker treatment conditions.

The results show that Ca/P ratio is sensitive to the process condition (in this case, treating time).

EXAMPLE 2

Whisker-Inducing Treatment of TTCP Particles Treated in Phosphate-Containing Acidic Solution

The procedures of Example 1 were repeated except that the basic aqueous solution was changed to 1M phosphorus acid aqueous solution having a pH of 0.8 and the immersion time was changed to 30 seconds.

Results:

Whiskers grown on TTCP surface are basic (Ca/P>1.33) in nature. The EDS-determined Ca/P molar ratios are between about 1.4 and about 3.7 with an average Ca/P ratio of about 2.0. Majority of whiskers have lengths <1000 nm and widths <200 nm.

EXAMPLE 3

Whisker-Inducing Treatment of TTCP Particles Treated in Phosphate-Free Basic Solution

The procedures of Example 1 were repeated except that the basic aqueous solution was changed to a basic aqueous NaOH solution having a pH of 10.66 and the immersion time was changed to 30 seconds and 24 hours.

Results:

-   (a) treated for 30 sec: No whisker was observed on TTCP surface. -   (b) treated for 24 h: Whiskers grown on TTCP surface are basic     (Ca/P>1.33) in nature. The EDS-determined Ca/P molar ratios are     between about 1.5 and about 3.4 with an average Ca/P ratio of about     2.0. Majority of whiskers have lengths <1000 nm and widths <300 nm.

EXAMPLE 4

Whisker-Inducing Treatment of TTCP Particles Treated in Phosphate-Free Acidic Solution

The procedures of Example 1 were repeated except that the basic aqueous solution was changed to 0.16M HCl aqueous solution having a pH of 0.8 and the immersion time was changed to 30 seconds, 10 minutes, one hour and 24 hours.

Results:

-   (a) treated for 30 sec: No whisker was observed on TTCP surface. -   (b) treated for 10 min: Whiskers grown on TTCP surface are basic     (Ca/P>1.33) in nature. The EDS-determined Ca/P molar ratios are     between about 1.4 and about 1.9 with an average Ca/P ratio of about     1.6. Majority of whiskers have lengths <200 nm and widths <100 nm. -   (c) treated for 1 h: Whiskers grown on TTCP surface are basic     (Ca/P>1.33) in nature. The EDS-determined Ca/P molar ratios are     between about 1.4 and about 1.9 with an average Ca/P ratio of about     1.6. Majority of whiskers have lengths <300 nm and widths <100 nm. -   (d) treated for 24 h: Whiskers grown on TTCP surface are basic     (Ca/P>1.33) in nature. The EDS-determined Ca/P molar ratios are     between about 1.5 and about 2.7 with an average Ca/P ratio of about     1.8. Majority of whiskers have lengths <1000 nm and widths <300 nm.     The diffraction patterns indicate these whiskers have a TTCP crystal     structure.

EXAMPLE 5

Compressive Strength of CPC Prepared from the Whisker-Grown TTCP Particles

Ca₄(PO₄)₂O (TTCP) powder as synthesized was sieved with a #325 mesh and has an average particle size of about 10 μm. To the sieved TTCP powder a HCl aqueous solution having a pH of 0.8 was added according to the ratio of 1 gm TTCP/13 ml solution. The sieved TTCP powder was immersed in the HCl solution for 12 hours, filtered rapidly and washed with deionized water, and filtered rapidly with a vacuum pump again. The resulting powder cake was dried in an oven at 50° C. The dried powder was divided into halves, ground for 20 minutes and 120 minutes separately, and combined. A setting solution of diammonium hydrogen phosphate was prepared by dissolving 20 mg of diammonium hydrogen phosphate, (NH₄)₂HPO₄, in 40 ml deionized water. 100 g of the mixed ground powder and 35 ml of the setting solution were well mixed to form a paste, which was then filled in molds to form specimens for compression test. The specimens were removed from the molds 15 minutes after the mixing, and soaked in a Hanks' solution. The soaked specimens were removed from the Hanks' solution at various periods of soaking time, and were immediately subjected to the compression test without drying. The compression test was conducted according to a method commonly used in the literature. The cylindrical samples have a diameter of 6 mm and a length of 12 mm.

Results: compressive strength is 27.4 MPa for the soaking time of 20 minutes, and 48 MPa for one-day soaking time.

EXAMPLE 6

Compressive Strength of CPC Prepared from the Whisker-Grown TTCP Particles

Ca₄(PO₄)₂O (TTCP) powder as synthesized was sieved with a #325 mesh and has an average particle size of about 10 μm. To the sieved TTCP powder the aqueous (NH₄)₂HPO₄ solution prepared in Example 1 was added according to the ratio of 1 gm TTCP/13 ml solution. The sieved TTCP powder was immersed in the (NH₄)₂HPO₄ solution for 5 minutes, filtered rapidly and washed with deionized water, and filtered rapidly with a vacuum pump again. The resulting powder cake was dried in an oven at 50° C. The dried powder was ground 120 minutes to obtain a powder A. The procedures in Example 5 were repeated to obtain a powder B except that the dried powder was ground only for a period of 300 minutes. A mixed powder of A and B in a ratio of 1:1 ratio was subjected to the compression tests following the procedures recited in Example 5.

Results: compressive strength is 26 MPa for the soaking time of 20 minutes, and 42.8 MPa for one-day soaking time.

EXAMPLE 7

Compressive Strength of CPC Prepared from the Whisker-Grown TTCP Particles

The procedures in Example 5 were repeated except that the HCl solution was changed to the aqueous (NH₄)₂HPO₄ solution prepared in Example 1 and the soaking time was changed to 5 minutes.

Results: compressive strength is 18.6 MPa for the soaking time of 20 minutes, and 48.8 MPa for one-day soaking time.

EXAMPLE 8

Compressive Strength of CPC Prepared from the Whisker-Grown TTCP Particles

Ca₄(PO₄)₂O (TTCP) powder as synthesized was sieved with a #325 mesh and ground for two hours. To the ground TTCP powder the powder B prepared in Example 5 was added and mixed in a ratio of 1:1. The resulting mixed powder was subjected to the compression tests following the procedures recited in Example 5.

Results: compressive strength is 19.7 MPa for the soaking time of 20 minutes, and 43.6 MPa for one-day soaking time.

EXAMPLE 9

X-Ray Diffraction of Whisker-Treated TTCP Powder and Immersed CPC Prepared from such TTCP

A TTCP powder was whisker-treated for 5 minutes according to the process described in Example 1. X-ray diffraction (XRD) was performed using an X-ray diffractometer (Rigaku D-max IIIV, Tokyo, Japan) with Ni-filtered CuK α radiation operated at 30 kV and 20 mA at a scanning speed of 1 degree/min. The phases were identified by matching each characteristic XRD peak with that compiled in JCPDS files.

Results: As indicated in FIG. 2, the XRD pattern of the whisker-treated TTCP powder (b) is substantially identical to that of TTCP as synthesized (a). The perfect match of every XRD peak position (diffraction angle) with the JCPDS data indicates that there is no additional phase formed during the whisker treatment. 0.7 g whisker-treated TTCP powder with 0.25 ml setting solution to form a CPC paste. The setting solution was prepared by dissolving 20 g (NH.sub.4).sub.2HPO.sub.4 in 40 ml deionized water. The CPC paste was filled in a cylindrical mold (12 mm in height and 6 mm in diameter), allowing hardening of the paste to occur within the mold. After 15 minutes the hardened CPC sample was removed from the mold and immersed in a 37.degree. C. Hanks' solution for 24 hours. After removing from the Hanks' solution and drying, the CPC sample was ready for XRD analysis. After immersion in Hanks' solution for 24 hours, the XRD pattern (c) of the CPC shows a large amount of HA phase which has replaced TTCP as the dominant phase. At this time only a small amount of TTCP remains. The result suggests that the CPC prepared from the whisker-treated TTCP powder of the invention can quickly transform into HA (the major component of human bone), once implanted.

Although a “basic” whisker can be grown on TTCP surface by immersion in a variety of solutions, the process should be carefully controlled. For example, when the solution contains a P source in the absence of Ca, the immersion time should be long enough to grow a basic whisker (an “acidic” whisker is grown at the early stage due to the excess P ions in the solution). Yet the immersion time should not be too long either to avoid the basic whisker's growing too large (should be within our claimed range), that can largely deteriorate the CPC properties.

On the other hand, when the solution does not contain P (e.g., HCl), acidic whisker is never grown on the surface of TTCP particles. All the observed whiskers on TTCP particles at all stages are basic in nature.

In addition to Ca/P ratio, the growth rate of a basic whisker is also sensitive to such process parameters as the type, pH, temperature and ion concentrations of the solution, to name a few.

Although the present invention has been described with reference to specific details of certain embodiments thereof, it is not intended that such details should be regarded as limitations upon the scope of the invention except as and to the extent that they are included in the accompanying claims. Many modifications and variations are possible in light of the above disclosure. 

1. A calcium phosphate cement prepared by the process comprising: contacting tetracalcium phosphate (TTCP) particles with a whisker-inducing solution to form whiskers or crystals comprising basic calcium phosphate having a Ca/P ratio greater than 1.33 on the surface of the TTCP particles; forming a paste by contacting the tetracalcium phosphate (TTCP) particles with a setting solution; and allowing the paste to form a hardened calcium phosphate cement.
 2. The calcium phosphate cement of claim 1, wherein the basic calcium phosphate whiskers or fine crystals are up to about 5000 nm long and up to about 500 nm wide.
 3. The calcium phosphate cement of claim 1, wherein the basic calcium phosphate whiskers or fine crystals are about 1 nm to about 2000 nm long, and are about 1 nm to about 200 nm wide.
 4. The calcium phosphate cement of claim 1, wherein the basic calcium phosphate whiskers or fine crystals are up to about 200 nm long, and are up to about 100 nm wide.
 5. The calcium phosphate cement of claim 1, wherein the basic calcium phosphate whiskers or fine crystals are up to about 300 nm long, and are up to about 100 nm wide.
 6. The calcium phosphate cement of claim 1, wherein the basic calcium phosphate whiskers or fine crystals have a Ca/P molar ratio in the range of about 1.4 to about 1.9.
 7. The calcium phosphate cement of claim 1, wherein the basic calcium phosphate whiskers or fine crystals have a Ca/P molar ratio in the range of about 1.4 to about 3.8.
 8. The calcium phosphate cement of claim 1, wherein the whisker-inducing solution comprises an acidic aqueous solution, a basic aqueous solution, an organic solvent, or substantially pure water.
 9. The calcium phosphate cement of claim 1, wherein the whisker-inducing solution comprises one or more of nitric acid (HNO₃), hydrochloric acid (HCl), phosphoric acid (H₃PO₄), carbonic acid (H₂CO₃), sodium dihydrogen phosphate (NaH₂PO₄), sodium dihydrogen phosphate monohydrate, sodium dihydrogen phosphate dihydrate, potassium dihydrogen phosphate (KH₂PO₄), ammonium dihydrogen phosphate (NH₄H₂PO₄), malic acid, acetic acid, lactic acid, citric acid, malonic acid, succinic acid, glutaric acid, tartaric acid, and oxalic acid.
 10. The calcium phosphate cement of claim 1, wherein the whisker-inducing solution comprises phosphoric acid (H₃PO₄).
 11. The calcium phosphate cement of claim 1, wherein the whisker-inducing solution comprises one or more of ammonia, ammonium hydroxide, alkali metal hydroxide, alkali earth hydroxide, disodium hydrogen phosphate (Na₂HPO₄), disodium hydrogen phosphate dodecahydrate, disodium hydrogen phosphate heptahydrate, sodium phosphate dodecahydrate (Na₃PO₄.12H₂O), dipotassium hydrogen phosphate (K₂HPO₄), potassium phosphate tribasic (K₃PO₄), diammonium hydrogen phosphate ((NH₄)₂(HPO₄), ammonium phosphate trihydrate ((NH₄)₃PO₄.3H₂O) and sodium bicarbonate (NaHCO₃).
 12. The calcium phosphate cement of claim 1, wherein the pH of the setting solution is in the range of 7.0 to 7.8.
 13. The calcium phosphate cement of claim 1, wherein setting solution comprises phosphate ions.
 14. The calcium phosphate cement of claim 1, wherein setting solution comprises ammonium phosphate.
 15. The calcium phosphate cement of claim 1, wherein at least one additive is included with the TTCP particles when the TTCP particles are contacted with the whisker-inducing solution.
 16. The calcium phosphate cement of claim 15, wherein the additive comprises one or more of sodium phosphate (Na₃PO₄), disodium hydrogen phosphate (Na₂HPO₄), sodium dihydrogen phosphate (NaH₂PO₄), disodium hydrogen phosphate dodecahydrate (Na₂HPO_(4.)12H₂O), disodium hydrogen phosphate heptahydrate (Na₂HPO₄.7H₂O), sodium phosphate dodecahydrate (Na₃PO₄.12H₂O), orthophosphoric acid (H₃PO₄), calcium sulfate (CaSO₄), Ca₄(PO₄)₂O, CaHPO₄.2H₂O, CaHPO₄, Ca₈H₂(PO₄)₆.5H₂O, α-Ca₃(PO₄)₂, β-Ca₃(PO₄)₂, Ca₂P₂O₇, and Ca₂H₂P₂O₈, (NH₄)₃PO₄, (NH₄)₂HPO₄ and (NH₄)H₂PO₄. 